1. Field of the Invention
This invention relates to a multi-cylinder internal combustion engine and, more particularly, to a split type internal combustion engine including a plurality of cylinders split into two groups and operable in a split-cylinder mode where one group of cylinders are held operative while the other group of cylinders are held suspended under engine low load conditions.
2. Description of the Prior Art
FIG. 1 is a schematic view of a conventional split type internal combustion engine. The engine comprises an engine body 1 containing therein a plurality of cylinders split into first and second groups, an intake passage 2 provided therein with a throttle valve 3 and divided downstream of the throttle valve 3 into first and second branches 2a and 2b, and an exhaust passage 4 provided with a three-way catalyzer (not shown) for purifying exhaust emissions. The first branch 2a communicates with the first group of cylinders #1 to #3 and the second branch 2b communicates through a stop valve 5 with the second group of cylinders #4 to #6. The second group of cylinders #4 to #6 are bypassed an exhaust gas recirculation (EGR) passage 6 provided therein with an EGR valve 7.
Under high load conditions, the stop valve 5 is open to allow fresh air to flow into the second group of cylinders #4 to #6 and the EGR valve 7 is closed to preclude re-introduction of exhaust gases into the second group of cylinders #4 to #6 so that the engine can operate in a full-cylinder mode where all of the cylinders are supplied with fuel and fresh air. When the engine is under low load conditions, the stop valve 5 is closed to block the flow of fresh air into the second group of cylinders #4 to #6 so that the engine can operate in a split-cylinder mode where the second group of cylinders are supplied with neither fuel nor fresh air. Under low load conditions, the EGR valve 7 is open to allow re-introduction of a portion of exhaust gases into the second group of cylinders so as to suppress pumping loss therein. Since the re-introduced exhaust gases are discharged from the suspended cylinders #4 to #6 during the split-cylinder mode of operation of the engine, the three-way catalyzer is held at a high temperature conductive to its maximum performance.
One difficulty with such a split-type internal combustion engine is that when the engine is shifted from a split-cylinder mode to a full-cylinder mode, the exhaust gases, which are re-introduced and filled in the second branch 2b of the intake passage 2 during the split-cylinder mode of operation, are drawn through the stop valve 5 into the first branch 2a since the second branch 2b is held substantially at atmospheric pressure due to recirculation of exhaust gases in amounts sufficient to suppress pumping loss in the suspended cylinders. This would cause miss fire in the first group of cylinders #1 to #3. However, any attempt to reduce the amount of exhaust gases recirculated into the second branch 2b so as to equalize the vacuum levels in the first and second branches 2a and 2b will cause an increased pumping loss and thus a fuel economy penalty. Furthermore, the filled exhaust gases are drawn into the second group of cylinders #4 to #6 to cause temporarily miss fire and rapid engine torque reduction just after the engine is shifted from a split-cylinder mode to a full-cylinder mode. This results in poor driving feel with shock and engine stalling if the engine is at low speeds.